Radio and television (T.V.) receivers of the superheterodyne type employ tunable local oscillators to convert all incoming radio frequency (R.F.) signals to a predetermined narrow intermediate frequency (I.F.) range. The I.F. section of a receiver can thus be optimally designed, because it always operates in that narrow frequency range regardless of which station or channel the receiver is tuned to. Because of this narrow-band operation, such I.F. sections commonly employ bandpass filters which are sharply tuned to the intermediate frequency.
In the T.V. field, such I.F. bandpass filters are often of the surface acoustic wave type. These devices have the advantage of being manufacturable by integrated circuit techniques. They comprise a substrate which is usually made of a piezo-electric material (although other materials have been employed; see U.S. Pat. Nos. 3,609,252 and 3,678,305). Upon the substrate are an input transducer (i.e., one which is connected to the electrical input signal) and an output transducer (i.e., one which produces the electrical output of the filter). These transducers each consist of interdigitated electrically conductive elements connected by pairs of bus bars. Both the elements and the bus bars are printed on the surface of the substrate. The spacing of these elements determines the operating frequency of the filter. See "Acoustic Surface Waves" by Kino and Shaw, Scientific American, October 1972, p. 50, which described an I.F. filter of this type developed for broadcast T.V. reception by a group including one of the present inventors.
When the elements are of uniform length and equally spaced, such interdigital (I.D.) transducers are quite frequency-selective. Each separate pair of elements in the input transducer across which a signal voltage is connected generates a weak acoustic wave. It is only by providing a plurality of such acoustic generator pairs, properly spaced to achieve acoustic phase coherency, that significant signal levels can be achieved. Any given pair spacing meets the conditions for phase coherency only near a specific frequency or an odd harmonic thereof. Therefore such an input transducer inherently has a very selective frequency response characteristic. If the output transducer likewise has uniform length elements and a fixed pair spacing, it also can produce a significant output level only when energized at an acoustic frequency for which that spacing produces phase coherency. A surface wave device in which both the input and output transducers are designed to operate at the same fundamental frequency is therefore a highly selective filter.
Such filters, however, do have spurious responses near the odd harmonics of their fundamental frequency. This has on occasion caused problems in T.V. receivers, particularly when interference near the third harmonic of the intermediate frequency was not rejected by the surface acoustic wave filter, and did adversely affect picture quality. Thus, there is a need to immunize S.W.I.F. devices from interference generally, and particularly near the third or other odd harmonics of the I.F. used in T.V. receivers.
The input transducer of a S.W.I.F. device directs its acoustic wave output in a fairly narrow beam or "track". The output transducer is also quite directional, and receives only acoustic signals arriving on its "track". If the two transducers are arranged so that their tracks coincide, they will be acoustically coupled so that surface (Rayleigh) waves generated by the input transducer are received by the output transducer.
This coincident track arrangement, however, may also cause the output transducer to pick up undesired bulk mode acoustic waves. The latter come from the input transducer and travel in the direction of the input track, but instead of staying on the surface of the substrate they pass through the body of the substrate by way of an internally reflected path. In order to discriminate between surface waves and bulk modes, it is a common practice to put the two transducers on two separate but parallel tracks. A track coupler is then provided on the surface of the substrate to shift surface acoustic energy from the input track where it is generated by the input transducer to the output track where it can be received by the output transducer. The track coupler, being a surface wave device, has no track coupling effect on the bulk wave. The latter, therefore, remains aligned with the input track where it cannot be detected by the output transducer. See, for example, "Surface Wave Filters," edited by Herbert Matthews, Chapter 6, "Surface Wave Bandpass Filter," by A. DeVries, published by John Wiley, New York City.
A known form of track coupler comprises a plurality of spaced electrically conductive strip elements without any bus bars connecting them together (i.e. a "multi-strip coupler" or M.S.C.). If the strips extend transversely across the width of two tracks, acoustic energy will be transferred laterally from one track to the other at certain spatially periodic locations. If the output transducer is located at one of the peaks of this spatial pattern, energy which starts out on one track can be effectively recaptured on the other.
There are some similarities, and also some important differences, between the operation of an M.S.C. and that of an I.D. transducer. Each neighboring pair of coupler strips of an M.S.C. forms an elemental transducer. An incoming acoustic wave induces a potential between these strips on the track where that wave arrives. Because the strips are conductive, the same potential exists all along the strip length, including the parallel track, where the potential is effective to regenerate acoustic energy. As in the design of an I.D. transducer, an M.S.C. employs multiple elements so that their individual outputs add up to useful levels. But an M.S.C., unlike an I.D. transducer, is not limited to a narrow frequency range determined by the element spacing.
As described by Marshall, Newton and Paige in their paper, "Theory and Design of the Surface Acoustic Wave Multistrip Coupler," I.E.E.E. Transactions on Sonics and Ultrasonics, April, 1973, p. 124, an M.S.C. operates over a broad band of frequencies. The mode of operation of an M.S.C. is different in this respect from that of an I.D. transducer. Because the M.S.C. does not have bus bars tying the voltages of alternate elements together, the phase relationship between them is governed by the incoming acoustic wave. Consequently the elements of an M.S.C. automatically match their phase relationships to that of the incoming wave even over a range of acoustic wavelengths which do not have a sharply defined relationship to the element spacing. The coupling efficiency of an M.S.C. for a given frequency does vary as a function of element spacing, but it does not drop off dramatically as in the case of a narrow band I.D. transducer. Therefore within certain limits an M.S.C. is quite tolerant of frequency changes.
An upper frequency limit for the M.S.C. is reached, in the view of Marshall, et al., supra, only when the operating frequency f approaches a value f.sub.o =v/2d, where v is the surface acoustic wave velocity of the substrate and d is the spatial periodicity of the M.S.C. elements.
Thus the choice of element periodicity in a M.S.C. has not been thought of as a design consideration in terms of center frequency, as it is in the case of an I.D. transducer. It has been thought of as a design consideration only in terms of the upper frequency cut-off. When the f.sub.o =v/2d condition is approached, acoustic reflections bounced back along the input track from the elements of the M.S.C. toward the input transducer begin to add constructively. As a result, too much of the acoustic energy is reflected and not enough is coupled to the output track. This blocks the operation of the S.W.I.F. device.
This invention recognizes that there is another gap in the coupler response when the spacing of the elements thereof is an integral multiple of an acoustic signal wavelength (i.e., d=I.lambda. where I is any positive integer). Under this condition (which corresponds to a frequency in excess of f.sub.o) the coupler has zero track coupling efficiency. Superficially, this appears to be a stopband similar to that of Marshall, et al., resulting from the periodicity of the M.S.C. elements. But, after careful analysis, it is realized that the effect upon which this invention rests is fundamentally different from the stopband effect previously described by Marshall et al. Marshall's f.sub.o =v/2d stopband results from the effect of echoes reflected backward along the input track. But the d=I.lambda. stopband recognized herein results from an entirely different phenomenon: the fact that the potential between neighboring fingers of the M.S.C. is zero at any frequency f for which the element spacing d is one, two, three, or any other integral number I times the surface acoustic wavelength. Moreover the stopband caused by this mechanism is typically much wider and deeper than that of Marshall et al.
With proper choice of the M.S.C. element spacing d, the filter can be designed so that the stopband conditions d=I.lambda. exist at or near any desired frequency f having a wavelength of .lambda.. If it is particularly desired to suppress the nth harmonic f.sub.n of a fundamental response frequency f.sub.s =f.sub.n /n, then d would be set equal to I.lambda..sub.n, where .lambda..sub.n is the wavelength of the nth harmonic and equals v/f.sub.n. But it is important to note that even when stopband conditions d=I.sub.n .lambda..sub.n exist for the nth harmonic frequency f.sub.n, stopband conditions d=I.sub.s .lambda..sub.s need not exist for the signal frequency f.sub.s itself. Thus the filter can discriminate effectively between the signal frequency f.sub.s and a frequency at or near its nth harmonic f.sub.n. If a filter for a T.V. receiver, d can be chosen so that f.sub.s is the intermediate frequency, and f.sub.n is the third harmonic f.sub.3 =3 f.sub.s. The filter will then reject the 3rd harmonic without rejecting the I.F. signal.
These and other features, objects and advantages of the invention will be more fully appreciated from the detailed description of the invention which follows, when read in conjunction with the drawings which accompany this specification.